The application of metallic coatings to glasses is widely employed to impart desirable physical properties such as thermal and electrical conductivity or reflectivity thereto, and many processes for applying such coatings to glasses are known.
Thick, thermally-conductive coatings on glass cooking vessels are particularly desirable to obtain improvements in thermal conductivity over the relatively poor conductivity exhibited by glasses alone. One method of applying thermally-conductive aluminum coatings to glass articles is described in U.S. Pat. No. 2,053,923, and involves the spray application of molten aluminum to a hot glass surface to obtain an adherent, conductive coating.
More recently, semicrystalline glasses or glass-ceramics have been utilized to produce cookware of improved strength and thermal durability, but again the low thermal conductivity of these materials as compared to metals has made the use of metallic coatings in combination therewith attractive. U.S. Pat. No. 3,523,013 describes one method of providing a glass-ceramic container with a metallic thermally-conductive coating.
The principal problems in the art of metal-coating glass-ceramic vessels arise out of the substantial differences in thermal expansion behavior between the conventional glass-ceramic materials used for the fabrication of cookware and common metals such as aluminum and copper which might be considered for use as coatings in combination with these glass-ceramics. For example, glass-ceramic materials typically employed for cookware fabrication exhibit rather low coefficients of thermal expansion, e.g., on the order of 10-25 .times. 10.sup.-7 /.degree. C, whereas aluminum, which has a desirable thermal conduction capability, has a coefficient of thermal expansion of about 230 .times. 10.sup.-7 /.degree. C. Theoretical stresses which may arise as a result of this expansion mismatch over the typical temperature range of use of an aluminum metal-glass-ceramic composite cooking vessel approach 100,000 psi, a stress level well above the failure point of both the glass-ceramic substrate and the aluminum coating. Increases in the thermal expansion of the glass-ceramic material to alleviate this expansion mismatch are not possible without sacrificing the excellent thermal shock resistance of this material, a major desirable feature of glass-ceramic cookware.
Prior art processes of applying aluminum or other conductive metal coatings to glass-ceramic vessels are disadvantageous for several reasons. First, multiple coating steps have been employed in order to achieve the desired degree of bonding together with a thickness providing a useful level of thermal conductivity. Secondly, metal application under controlled neutral-atmosphere conditions has been required. And finally, molten metal application has typically been carried out only by applying the molten metal to very hot glass-ceramic surfaces, in order to achieve the degree of bonding which was thought to be required.
Aluminum-coated cooking vessels prepared in the conventional manner may exhibit a variety of undesirable characteristics in the course of use over an extended period of time. Repeated thermal cycling may eventually produce delamination of the aluminum coating, affecting the appearance and thermal performance of the product. Also, occasional severe thermal downshock which is experienced by a hot vessel transferred from the stovetop to cold water can cause immediate failure of the vessel because of the very high stresses generated during rapid, non-uniform cooling. Whereas these problems are not too severe with thin (1-10 mil) coatings, thicker coatings, on the order of 20 mils, which are required to achieve improved heat distribution, substantially increase the likelihood of thermal shock breakage.
Accordingly, it is a principal object of the present invention to provide an aluminum-coated glass-ceramic cooking vessel suitable for stovetop use which combines the attractiveness and thermal shock resistance of glass-ceramic cookware with the desirable heating characteristics of metal cookware.
It is another object of the present invention to provide an improved process for manufacturing aluminum-coated cookware which overcomes many of the disadvantages of prior art processes for producing such ware.
Other objects and advantages of the invention will become apparent from the following summary and detailed description thereof.